Two-layered photoconductive element containing a halogen-doped storage layer and a selenium alloy control layer

ABSTRACT

A xerographic plate having a double layered photoconductive portion, the lower layer being designated as a storage layer and the upper layer a control layer. The storage layer consists of halogen doped selenium in a thickness from about 20 to 200 microns. The control layer consists of undoped selenium alloys in a thickness of about 0.1 to 5 microns. The plate utilizes the optimum photoconductive properties of each layer.

United States Patent [151 3,639,120

shelling Feb. 1, 1972 [s41 TWO-LAYERED PHOTOCONDUCTIVE 2,803,541 8/1957Paris ..96/l

ELEMENT CONTAINING A HALOGEN- g DOPED STORAGE LAYER AND A my 3,312,5474/1967 Levy .96/l.5

SELENIUM ALLOY CONTRGL LAYER 3,312,548 4/1967 Straubhan ..96/l.5

Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, lll

Attorney-Frank A. Steinhilper, Stanley 2. Cole and James J. Ralabate [57] ABSTRACT A xerographic plate having a double layered photoconductiveportion, the lower layer being designated as a storage layer and theupper layer a control layer. The storage layer consists of halogen dopedselenium in a thickness from about 20 to 200 microns. The control layerconsists of undoped selenium alloys in a thickness of about 0.1 to 5microns. The plate utilizes the optimum photoconcluctive properties ofeach layer.

9 Claims, 3 Drawing Figures PATENTEDFEB H97? $639,120

POTENTIAL (VOLTS) I I l o 0.4 0.8 U2 U6 TIME (secouos) soo- /INIT|ALDISCHARGE POTENTIAL (VOLTS) loo /RESlDUAL DISCHARGE O l I 1 TIME(SECONDS)*** INVILN'IUR.

CHRISTOPHER SNELLING ATTORNEY TWO-LAYERED PHOTOCONDUCTIVE ELEMENTCONTAINING A HALOGEN-DOPED STORAGE LAYER AND A SELENIUM ALLOY CONTROLLAYER This invention relates to xerography, and more specifically, to asystem employing a xerographic plate having a novel storage layer.

In the art of xerography, a xerographic plate containing aphotoconductive insulating layer is first given a uniform electrostaticcharge in order to sensitize its surface. The plate is then exposed toan image of activating electromagnetic radiation such as light, X-ray,or the like which selectively dissipates the charge in the illuminatedareas of the photoconductive insulator while leaving behind a latentelectrostatic image in the nonilluminated areas. The latentelectrostatic image may be developed and made visible by depositingfinely divided electroscopic marking particles on the surface of thephotoconductive insulating layer. This concept was originally disclosedby Carlson in U.S. Pat. No. 2,297,691, and is further amplified anddescribed by many related patents in thefield.

The discovery of the photoconductive insulating properties of highlypurified vitreous selenium has resulted in this material becomingstandard in commercial xerography. Vitreous selenium, however, issomewhat limited in its spectral response which is very largely limitedto the blue or near ultraviolet portion of the spectrum. Seleniumphotoconductive coatings are also subject to abrasive wear over longrange cyclic use, and under conditions of high humidity with or withoutabrasive wear, the selenium exhibits poor printing properties due tolateral surface conductivity.

It is also well known that the light discharge characteristics ofselenium, and plates employing a thin layer of a more sensitivephotoconductor over selenium, both exhibit a pronounced knee" beyondwhich the rate of discharge (loss of voltage per unit time under a givenexposure to a source of activating radiation) is drastically reduced.

With the above problems in mind, the art has looked for ways to improveboth the photosensitive and physical properties of vitreous selenium. InUS. Pat. No. 2,860,048, to Deubner, a photoconductive insulating layersuch as selenium, is protected by a thin coating of about 1 micron inthickness of an organic material which improves both humidity stabilityand abrasion resistance. US. Pat. No. 2,901,348, to Dessauer et al.discloses a plate structure wherein conventional photoconductive layerssuch as vitreous selenium is sandwiched between two barrier layers toprevent any charge leakage from the plate prior to exposure toactivating radiation. US. Pat. No. 2,90l ,349, to Schaffert et al.contemplates a conventional selenium layer which overlays a layer ofarsenic trisulfide the purpose of. which is to render the plate capableof being charged both positively and negatively without loss of spectralsensitivity and also to improve fatigue resistance. It is alsocontemplated that the photoconductive part of the plate may consist of aplurality of discrete layers each containing a different type ofphotoconductive insulating material. One suggested combination is avitreous selenium layer on top of which is placed a mixture of telluriumand selenium. It can thus be seen that the art of commercial xerographyhas structurally modified the basic conventional selenium plate so as toimprove both its photosensitive and physical properties.

it is, therefore, an object of this invention to provide a xerographicsystem which overcomes the above-noted disadvantages.

It is another object of this invention to provide a xerographic platehaving improved physical properties.

It is a further object of this invention to provide a system utilizing axerographic plate having improved discharge characteristics.

it is yet a further object of this invention to provide a xerographicplate having maximum photosensitivity and charge dischargecharacteristics and yet which can be made by conventional techniques.

The foregoing objects and others are accomplished in accordance withthis invention by preparing a xerographic plate having a doublelayeredphotoconductive portion comprising a lower storage layer which comprisesvitreous selenium doped with a halogenand a relatively thin controllayer overlaying such storage layer comprising an undoped vitreousselenium alloy having greater photosensitivity than said storage layer.

The advantages of this invention will become apparent upon considerationof the following disclosure of this invention; especially when taken inconjunction with the following drawing wherein:

FIG. 1 is a schematic illustration of one embodiment of a xerographicplate as contemplated by this invention.

FIG. 2 graphically illustrates the discharge properties of threedifferent xerographic plates.

FIG. 3 graphically illustrates the discharge characteristics of thenovel plate of this invention.

FIG. 1 shows an improved xerographic plate 10 according to thisinvention. Reference character lll designates an elec' tricallyconductive substrate or mechanical support. This is conventionally ametal such as brass, aluminum, gold, platinum, steel or the like. Thesupport member may be of any convenient thickness, rigid or flexible, inthe form of a sheet, web, cylinder, or the like, and may be coated witha thin layer of plastic. It may also comprise such other materials asmetallized paper, plastic sheets covered with a thin coating of aluminumor copper iodide, or glass coated with a thin layer of chromium or tinoxide. An important consideration is that the member be somewhatelectrically conductive or have a somewhat conductive surface and thatit be strong enough to permit a certain amount of handling. In certaininstances support 11 may even be dispensed with entirely. Referencecharacter 12 designates a storage layer which comprises conventionalhigh-purity vitreous selenium doped with a halogen such as chlorine,fluorine, bromine, or iodine. The halogen is present in relatively smallamounts which are measured in parts per million. For the purposes ofthis invention concentrations of from about 10 to 10,000 parts permillion of a halogen have been found effective in doping the seleniumlayer.

The selenium is conveniently purchased to specification with the desiredconcentration of dopant already present. Canadian Copper Refiners is asource of predoped selenium. if desired, the selenium may be doped byany conventional laboratory techniques such as physically mixing thedopant (such as iodine) with the selenium and vacuum evaporating themixture onto the conductive substrate. Bromine could be added in. theform of liquid drops to the selenium which is precooled. Chlorine orfluorine may be added by admitting chlorine or fluorine gas to anevacuated tube containing selenium (which is precooled) and maintainingthe flow of gas until the selenium contains the desired amount ofdopant. It should also be pointed out that the halogen may be added tothe selenium as a compound of the selenium.

Storage layer 12 may be in any suitable thickness used for conventionalphotoconductive layers. Typical thicknesses conveniently range fromabout 20 to 200 microns. A relatively thinner control layer 13 overlaysphotoconductive storage layer 12. Control layer 13 contains a moresensitive photoconductive material than said storage layer 12. Thecontrol layer may consist of a selenium-arsenic alloy containing up toabout 50 percent by weight arsenic, such as that shown by US. Pat. No.2,803,542, to Ullrich or US. Pat. No. 2,822,300, to Mayer et al., or aselenium-tellurium alloy containing up to about 30 percent by weighttellurium. This overlaying control layer should be in a thickness ofabout: 0.l to 5.0 microns in thickness. Thicknesses above about 5microns result in an undesirable high dark discharge and light fatigueconditions while thicknesses below about 0.1 microns fail to give asubstantial increase in photosensitivity.

Halogen doping a photosensitive member having a single photoconductivelayer throughout the layer would appear to be undesirable due to aresulting high dark discharge rate.

It can thus be seen that the plate of FIG. 1 divides the photoconductiveportion of the plate into two layers: a storage layer which functions tocontrol the field and thus control the discharge rate of the plate, andan overlaying control layer having a greater sensitivity than the lowerstorage layer. By thus combining the two separate layers, each layerfunctions to give a particular optimum property resulting in a novelxerographic plate having optimum optical properties for lightsensitivity together with a desirable high discharge rate.

FIG; 2 illustrates the substantial improvement in the electricalcharacteristics of a two-layered plate as contemplated by thisinvention. The light discharge characteristics of a conventionalselenium plate (curve A) having a 25 micron selenium layer overlaying analuminum substrate; a selenium-tellurium two-layered plate (curve B)having a 0.1 micron selenium-tellur layer overlaying a 25 micronselenium layer on an aluminum substrate; and a 25 micron chlorine dopedselenium storage layer, with a 0.1 micron selenium-tellurium layeroverlaying the storage layer, on an aluminum substrate (Curve C), areeach compared in regard to their discharge rate as shown in FIG. 2.

The plates of Curves B and C contain about percent tellurium in thecontrol layer, and the plate of Curve C is doped with 60 parts permillion of chlorine in the selenium storage layer.

The potential or voltage is plotted on the ordinant while exposure timeis plotted on the abscissa. It can be seen that the rate of discharge ofCurve A for the single layer selenium plate is further to the right thanCurve B or C. Curve B has a pronounced knee below which the rate ofdischarge is drastically reduced, which is above that for Curve C forthe selenium-tellurium overcoated plate using a chlorine doped seleniumstorage layer. This plate (Curve C) shows the most favorable dischargerate of the three plates and has a knee which is lower than both that ofthe selenium plate and the seleniumtellurium two layered plate of CurveB which is not doped with chlorine.

Although the theory is not completely understood, it is suggested thatthe knee" might be related to an unfavorable electric field reductionoccurring in the region of light absorption. For example, theestablishment of a space charge of trapped holes at a distance below thesurface might be occurring. As a remedial measure, the use of halogendoped selenium as a lower layer was used based upon experiments whichhave indicated a capability of controlling the local electrical field inthis material by virtue of electron trapping in thermal equilibrium. Itwas proposed that the decrease in electric field at the surface due tohole trapping might be offset by the increase due to the space charge ofelectrons trapped in the lower layer. The results as shown asillustrated by the discharge curve in FIG. 2 have been consistent withthis theory.

Two series of plates such as those illustrated by B and C of FIG. 2 wereprepared to demonstrate the utility of the chlorine addition to thelower layer. The plates are charged to a uniform positive surfacepotential of 300 volts and exposed to radiation bands of 4,500 and 5,775angstroms.

The term Vi in Table l is the voltage at which straight line extensionsof the initial discharge and residual discharge segments intersect whichis an approximation to knee voltage. This is illustrated by FIG. 3 ofthe drawings wherein Vi is shown for plate 1 of Table l. Two differentalloys of the same composition were used in each set with the data beingtabulated in Table l below. Each plate contains a micron thick storagelayer of selenium on an aluminum substrate, with a 0.1 micron controllayer of selenium-tellurium overlaying the storage layer. The storagelayer of plates 1 and 3 are doped with chlorine, while the storage layerof plates 2 and 4 are undoped.

TABLE I V|'(4500 angstroms) Vi(5750 angstroms) 20 volts Control layer0.] microns selenium-10% tellurium) Storage layer 25 micron layer ofselenium doped with 60 parts per million chlorine on an aluminumsubstrate 70 volts Control layer 0.1 microns of 90% selenium-10%tellurium Storage layer 25 micron layer of high purity selenium (nodoping) on an aluminum substrate 40-45 volts Control layer 0.l micronalloy of 90% seleniuml0% tellurium Storage layer 25 micron layer of 60parts per million chlorine on an aluminum substrate volts Control layer0.l micron layer of alloy 90% seleniuml0% tellurium Storage layer 25micron layer of selenium (no doping) on an aluminum substrate Plate l15-20 volts 60 volts Plate 2 Plate 3 30 volts Plate 4 85 volts It can beseen from Table I that chlorine doped plates 1 and 3 both shownapproximately three times the discharge (Vi) as undoped plates 2 and 4,at both wavelengths.

ln addition to reducing Vi by a factor of approximately 3x for halogendoped plates as opposed to undoped plates, an increase in measuredresponse is indicated by these data due to the inclusion of chlorine inthe storage layer. This effect is interpreted as a general straighteningof the discharge curve even above the knee.

The plates of this invention may be prepared by any of the well-knownconventional techniques such as those set forth in the above mentionedUllrich and Mayer et al., patents. Such techniques briefly involveforming an alloy such as selenium and arsenic by melting the appropriateamount of arsenic and selenium together in a temperature range ofapproximately 750-900 F. The resulting alloy is then evaporated undervacuum conditions onto the overlaying storage layer.

The storage layer is also evaporated onto the conductive substrate byany conventional techniques such as that shown by U.S. Pat. No.2,753,278, to Bixby et al., and U.S. Pat. No. 2,970,906, to Bixby. Ifdesired, both the storage layer and control layer may be evaporatedsequentially without breaking the vacuum. This avoids the possibledanger of contaminating the surface of the plate.

Although specific components and proportions have been stated in theabove description of the preferred embodiments of this invention, othersuitable materials and procedures such as those listed above may be usedwith similar results. in addition, other materials may be added to theplates which synergize, enhance, or otherwise modify their properties.

Various additions, such as sensitizers, may be added to enhance theproperties of the novel plate contemplated by this invention.

Other modifications and ramifications of the present invention wouldappear to those skilled in the art upon reading the disclosure. Theseare intended to be included within the scope of this invention.

What is claimed is:

1. A xerographic plate comprising a two-layered photoconductive portion,said first portion comprising a storage layer consisting of vitreousselenium doped with a halogen in a concentration of from about l0 to30,000 parts per million, and an overlaying photoconductive controllayer having a thickness of from about 0.1 to 5 microns comprising aphotoconductive material selected from the group consisting ofselenium-tellurium and selenium-arsenic alloys.

2. The plate of claim 1 wherein the storage layer ranges in thicknessfrom about 20 to 200 microns.

3. The plate of claim 1 wherein the halogen is chlorine.

4. A xerographic plate comprising:

a. a conductive substrate,

b. a vitreous selenium layer doped with a halogen in a concentration offrom about to 10,000 parts per million overlaying said substrate, and

c. a control layer having a thickness of from about 0.1 to 5 micronscomprising a photoconductive material selected from the group consistingof selenium-tellurium and selenium-arsenic alloys.

5. The plate of claim 8 wherein the halogen dopant is chlorine.

6. A xerographic plate comprising:

a. a conductive substrate having thereon b. an overlayingphotoconductive storage layer from about 20 to 200 microns thick,comprising vitreous selenium doped with chlorine in a concentration ofabout 10 to 10,000 parts per million,

0. a control layer in a thickness of from 0.1 to 5 microns in thicknessoverlaying said storage layer, said control layer comprising a moresensitive photoconductor than said storage layer, and saidphotoconductor consisting of a member selected from the group ofarsenic-selenium and selenium-tellurium.

7. An imaging method comprising:

a. providing a xerographic plate having a two-layered photoconductiveportion, said first portion comprising a storage layer consisting ofvitreous selenium doped with a halogen in a concentration from about 10to 10,000 parts per million, and an overlaying photoconductive controllayer, having a thickness of from about 0.1 to 5 microns comprising aphotoconductive material selected from the group consisting ofselenium-tellurium and selenium-arsenic alloys.

b. forming an electrostatic latent image of said plate, and

c. developing said image to make it visible.

8. A method of forming an electrostatic latent image which 10 comprises:

developed to make it visible.

2. The plate of claim 1 wherein the storage layer ranges in thicknessfrom about 20 to 200 microns.
 3. The plate of claim 1 wherein thehalogen is chlorine.
 4. A xerographic plate comprising: a. a conductivesubstrate, b. a vitreous selenium layer doped with a halogen in aconcentration of from about 10 to 10,000 parts per million overlayingsaid substrate, and c. a control layer having a thickness of from about0.1 to 5 microns comprising a photoconductive material selected from thegroup consisting of selenium-tellurium and selenium-arsenic alloys. 5.The plate of claim 8 wherein the halogen dopant is chlorine.
 6. Axerographic plate comprising: a. a conductive substrate having thereonb. an overlaying photoconductive storage layer from about 20 to 200microns thick, comprising vitreous selenium doped with chlorine in aconcentration of about 10 to 10,000 parts per million, c. a controllayer in a thickness of from 0.1 to 5 microns in thickness overlayingsaid storage layer, said control layer comprising a more sensitivephotoconductor than said storage layer, and said photoconductorconsisting of a member selected from the group of arsenic-selenium andselenium-tellurium.
 7. An imaging method comprising: a. providing axerographic plate having a two-layered photoconductive portion, saidfirst portion comprising a storage layer consisting of vitreous seleniumdoped with a halogen in a concentration from about 10 to 10,000 partsper million, and an overlaying photoconductive control layer, having athickness of from about 0.1 to 5 microns comprising a photoconductivematerial selected from the group consisting of selenium-tellurium andselenium-arsenic alloys. b. forming an electrostatic latent image ofsaid plate, and c. developing said image to make it visible.
 8. A methodof forming an electrostatic latent image which comprises: a. providing axerographic plate having a two-layered photoconductive portion, saidfirst portion comprising a storage layer consisting of vitreous seleniumdoped with a halogen in a concentration of from about 10 to 10,000 partsper million, and an overlaying photoconductive control layer having athickness of about 0.1 to 5 microns comprising a photoconductivematerial selected from the group consisting of selenium-tellurium andselenium-arsenic alloys, b. substantially uniformly electrostaticallycharging said plate, and c. exposing said plate to a pattern ofactivating electromagnetic radiation.
 9. The method of claim 8 whereinthe latent image is developed to make it visible.